The invention relates to a process for producing a field-effect transistor in which the respective positions of the source, the gate and the drain are fixed at the stage of the first masking operation, which entails correct positioning of the masks used in the following operations. In such a case it is said that there is self-alignment of the gate.
A self-alignment process in known in which a layer of aluminum is deposited on the active semiconductor layer intended to receive the three conventional electrodes (source, gate and drain). On this aluminum deposit is deposited a layer of resin. It is on this resin that the first masking operation is carried out: it consists in opening two windows defining the respective location and size of the source and of the drain, and in leaving between the windows a cap of resin which determines approximately the location of the gate. The size of this gate, and its final position, are determined by prolonged etching of the aluminum until there is obtained, by sub-etching, a stub of aluminum covered by the resin cap.
This cap serves to protect the location of the grid during depositing of the source and drain contacts by metal evaporation.
The process is then impaired by inaccuracy in the location and the dimensions of the gate, especially when this latter is to be very fine, and very close to the source and the drain, as is necessary in very high frequencies. This accuracy is only partially improved by using X-rays in the masking operations. In fact, though the cap is then defined with greater precision, there still remains a lack of precision tied to the more or less difficult control of the chemical etching which follows the first masking operation.